The present invention relates generally to the monitoring of output power of a semiconductor laser diode and, particularly, to the monitoring of the output power of a dual beam laser diode used in a Raster Output Scanning (ROS) system.
It is well known in the scanning art to use laser diodes to generate a coherent laser beam which is optically shaped and used to scan in a ROS system. It is also known to use multiple laser diodes to create multiple beams, each individual beam independently modulated by video signals, and the multiple beams scanned onto the recording surface as modulated beams. For these multiple beam applications, it has been found advantageous to use arrays of closely spaced laser diodes. Closely spaced diodes allow for multiple beam processing and thus improve data throughput as compared with systems that employ continuous wave, single beam gas or laser diodes.
Typically, the laser diodes in a multiple beam system are individually addressable. Individual addressability generally requires that each diode have a separate current source that drives or modulates the diode. In operation, each driver sends a current through the diode sufficient to induce emission of laser light. The amount of current the driver produces is determined, in part, by the digital input data driving that particular lasing element. An example of a Raster Output Scan (ROS) system using a dual laser diode is disclosed in U.S. Pat. No. 4,796,964, whose contents are hereby incorporated by reference.
Because different laser diodes have different output power characteristics in response to a given driving current, it is desirable to monitor the amount of output power from each laser diode.
The requirement imposed on output power monitoring is that the light output from a first diode must be detected independent of the light output of the second diode. One prior art method for monitoring and controlling the output power of dual beam laser diodes is to detect radiation emitted from the back facet of each laser diode as shown in FIG. 1.
FIG. 1 shows a side schematic view of a prior art ROS system utilizing back facet detection feedback for controlling the diode output. Input video data is transmitted to a self-modulating light source 12, such as a low powered solid state laser diode, which produces a modulated diverging beam of coherent light. The diode is driven in accordance with image signals entered into and processed by ESS 17. The beam is collimated by a spherical collimating lens 14 and is next incident upon a cylindrical lens 15 which focuses the light to a line image in the fast scan direction onto a rotating polygon 20 having at least one mirrored facet 21. The rotation of the mirrored facets causes the beam to be deflected and thereby scanned across a photosensitive image member which is shown as a photoreceptor drum 24. Postscan optics system 22 reconfigures the beam reflected by facet 21 to a circular or elliptical cross-section, refocuses the beam to the proper point on the surface of drum 24, and corrects for scan nonlinearity (f-theta correction). A 1X (or other working magnification) toroidal lens 28 or cylinder mirror (not shown) is disposed between the scanning device 20 and the photoreceptor 24 to correct for wobble (scanner motion or facet errors) where appropriate.
The laser diode 12 has front and back facets. While the majority of the laser light escapes from the front facet, some radiation is emitted from the back facet of the diode. This radiation is detected by a photodiode 32 which generates output signals which are sent into feedback circuit 44. This signal is compared to a predetermined voltage level corresponding to the desired power output of the laser diode. If correction is needed, a signal is sent to the laser diode drive circuit to increase or reduce the emitter power output.
There are several problems with this prior art system. One is a slow response time of the detection and feedback system which makes real time, pixel by pixel control which is necessary to eliminate exposure variations along the scan line impossible at high printing rates. Adding to the real time control problem is the restriction of a single photodiode monitoring the output from one of the laser diodes while the other laser diode is being modulated. These problems, inherent in a back facet detection system, have been addressed in U.S. Pat. No. 5,600,126, issued on Feb. 4, 1997 and assigned to the same assignee as the present invention. In this application, two back facet photodiodes were used. The polarization of the laser diode back facet light output was controlled so that each photodiode would be separately and simultaneously measuring the light intensity of the associated laser diode. A preferred feedback circuit was also described which provided a fast response time and an improved control of each laser diode output power level.
While the back facet detection power output control method provides satisfactory results, one disadvantage is the need for very accurate calibration of the photodiodes since they are detecting only a small fraction (about 0.5%) of the total laser diode output. It would be desirable to control the laser diode output power levels by using photodiodes positioned in the output beam path of each laser diode, that is in the path of the light emitted out of the front facet of the laser diode. This implementation would require isolation of each photodiode, both from the laser diode not being instantly monitored as well as stray light from optical components located in the pre-polygon optical system.
Thus, there is a need to construct an output power detection system such that the amount of light emitted from individual emitters, in a multiple laser diode configuration, is detected by corresponding photodiodes.
It is thus a first object of the present invention to provide a monitoring system such that the amount of output power from individual emitters of laser diodes can be individually monitored in a continuous fashion with a relatively fast response time.
It is a further object of the invention to reduce the effects of stray light interference on photodiodes positioned in the pre-polygon optical system.
These and other objects are realized by positioning two small photodiodes relatively close to the front or emitting end of the laser diodes being monitored. An optical element is positioned in the pre-polygon optical system so as to reflect a portion of the laser diode output radiation back towards the photodiodes. In one embodiment, the photodiodes are mounted on the laser diode heat sink assembly and light is reflected back onto the photodiodes by a tilted beam splitter.
In another embodiment, the photodiodes are positioned adjacent to the collimator lens and light is reflected back to the photodiodes by an angled, curved aperture plate. More particularly, the present invention is related to a scanning optical system for scanning an image plane with a modulated laser beam from at least one laser diode, the laser diode providing a modulated output beam at a predetermined power level, the output beam collimated by a collimator lens and scanned by a beam scanner through beam shaping optics and onto the imaging plane, the scanning optical system further including monitoring means for monitoring and adjusting the laser diode power level, said monitoring means including:
an optical reflective element positioned between the collimator lens and the beam scanner so as to reflect a portion of collimated light back towards the laser diodes, PA1 detector means positioned so as to intercept at least a portion of said light reflected from said optical reflector element and to generate an output signal related to the amount of detected light, and PA1 feedback circuit means for monitoring the photodiode output signals and for adjusting the laser diode output so as to maintain the output at said predetermined power level.